|
Hugh,
I don't know the science behind it, but you will notice that higher horsepower engines seem to have more prop blades. The Malibu/Mirage is the only plane I can think of that has >300HP with only two blades.
I assume that you can only bite so much air per blade (i.e. increasing the angle of attack further results in an aerodynamic stall) - and therefore to get the advantage of the higher horsepower, you need more blades to bite the air. Another choice appears to be simply increasing the size of the blades (check out typical warbirds), but then ground clearance becomes an issue.
I've heard that one way to pass the much stricter European noise limits is to use a smaller diameter prop with more blades. The tips of the prop are aerodynamically going the fastest and create much of the noise. By reducing the diameter you can slow down the tip speed - and the resulting loss of thrust is replaced with an additional blade.
|